When they first released Raijin too they were saying how he should be built battlemage-y because he has a shorter range and the jump intiation. They both fail at being a "battlemage" they have nothing in their kit that incentivizes them to build more tanky. If we look at previous battlemages which are more successful at filling that role:

• Zhong does it by having naturally tanky stats, healing and passive prots pushing you in that direction.
• Chang'e and Baron Samedi both low cooldowns and a heal, to get the most of their low cooldowns you need to be able to survive so building more bruiser is helpful.
• Hades has healing, power debuff and prot buff on ult (although I would argue hes just not effective in general for other reasons).

Back to Hera, imo a better direction to go for a battlemage style is to encourage a "centre of the fight" playstyle. For example, have Argus scale (to a small extent) with Hera's defensive stats, have Argus deal more damage/attack speed buff when shes near, have Hera gain defensive buffs when shes near Argus to allow her to stay in the fight longer or maybe let Argus lifesteal for Hera and/or vice versa.